These pairs of books bear the same book
numbers. I'm not sure what prompted the screw-up, but it may have had
something to do with the implementation of the "K" suffix (mentioned in a
previous "Oddity"), which first took effect with #267.

To my knowledge, these are the only
duplicated numbers in the Ballantine run.

The Bilko Joke Book was supposed to be
numbered 298K, according to Holroyd (but that may have been an error on
his part). I have never seen a copy or a scan of a book bearing that
number, and I am assuming that none exists.

As for the two books numbered 269:
there are no known Ballantine numbers 270 or 271. It is probable that The Key to Economic Progress was originally meant to bear one
of those numbers. It is not listed in any vintage paperback price
guide. (I own that book, and there is absolutely no doubt that is the
correct number.)

Both scans of 289K are courtesy of Daniel Snethen.

The scan of 269K is courtesy of Brian Maginnity.

The scan of S476K is from Dave Biemann, who pointed out
yet another number duplication.